This invention relates to a display control system.
Conventionally, image display is controlled by one of the following systems shown in FIGS. 5(a), (b ) and (c).
The display control system shown in FIG. 5(a) is a so-called software window system involving raster operation. In this system, the image data of the windows A, B and C stored in a window memory 1 are transferred in blocks to a display memory 2 through raster operation so that picture editing such as positioning and superposing of the windows A, B and C are performed in the display memory 2. After editing, the image data are read sequentially from the display memory 2 for multi-window display on a CRT 3.
The display control system shown in FIG. 5(b) is a so-called hardware window system involving a mapping table. In this system, the address of the image data corresponding to the current scanning position on a CRT 6 is output sequentially from a hardware mapping table 5 during scanning operation by the CRT 6, and the image data of the windows A, B and C stored in a window memory 4 are read in shared time according to the above address and output directly to the CRT 6 for multi-window display.
The display control system shown in FIG. 5(c) is a so-called software window system involving clipping. In this system, a picture is drawn in a display memory 8 using the code data for the image information of the windows A, B and C stored in a segment buffer 7 after clipping the code data of the image information outside the windows. The image data is then read sequentially form the display memory 8 for multi-window display on a CRT 9.
The above conventional display control systems of FIGS. 5(a), (b) and (c) have disadvantages.
The disadvantage of the display control system of FIG. 5(a) involving raster operation is as follows. The image data in the window memory 1 must be transferred in blocks to the display memory 2 to edit a picture in the display memory 2 before the picture is displayed on the CRT 3. This operation must be carried out every time the window is moved on the CRT 3. Therefore, the window cannot be moved quickly.
The disadvantage of the system of FIG. 5(b) involving a mapping table is as follows. The addresses of the image data in the window memory 4 are output sequentially from the mapping table 5 so that the image data stored at the addresses are read in shared time and displayed directly on the CRT 6. With this system, it is possible to move the window quickly. On the other hand, however, since graphic drawing in the window memory 4 is also performed in shared time, drawing speed is slow.
The disadvantage of the system of FIG. 5(c) involving clipping is as follows. The code data of unnecessary image data in the segment buffer 7 is removed by clipping before the code data of the image data stored in the segment buffer 7 is transferred for graphic drawing on the display memory 8 and displayed on the CRT 9. To move the window on the CRT 9, therefore, the system requires hardware for drawing graphics at a high speed on the display memory 8 from the code data stored in the segment buffer 7.